# Patent

## Title
SRAM bit cell with reduced bit line pre-charge voltage

## Abstract

An SRAM bit cell comprises a first inverter including a PMOS transistor and an NMOS transistor, and a second inverter including a PMOS transistor and an NMOS transistor. The first and second inverters are cross-coupled to each other. A plurality of pass transistors couple the inverters to bit lines. Approximately one-half of a supply voltage is provided to the bit lines during pre-charge operations.

## Background
BACKGROUND
1. Field
This disclosure relates generally to semiconductor memory devices, and more specifically, to static random access memory (SRAM) devices.
2. Related Art
Semiconductor memory devices are used in a wide variety of electronic devices such as portable telephones, digital cameras, mobile and stationary computing devices, automobiles, appliances, and other devices.
The key attributes of an SRAM memory are the speed and power of the memory. SRAM memory is often employed when high speed access is required. SRAM memory is a static memory that requires power to be maintained for data retention. However, data retention requires relatively little power. Much of the power dissipated in an SRAM memory relates to the bit lines charging and discharging. Another important aspect of an SRAM is the signal provided to the bit lines by a bit cell during a Read access, that is, how fast that signal is provided and how reliably that signal can be sensed. Another key attribute of an SRAM memory is the soft error rate, that is, the ability of the memory to retain data when exposed to high energy sub-atomic particles. Therefore there is a need for an SRAM memory bit cell that requires lower power, that is faster to provide a signal that can be sensed reliably and that has a lower soft error rate.



BRIEF DESCRIPTION OF THE DRAWINGS
The present disclosure is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.
FIG. 1 illustrates a block diagram of an embodiment of a processing system in accordance with the present invention.
FIG. 2 illustrates a block diagram of components that can be included in SRAM memory of FIG. 1 in accordance with the present invention.
FIG. 3 illustrates a circuit diagram of a bit cell that can be included in SRAM memory of FIG. 2.
FIG. 4 illustrates a circuit diagram of an embodiment of a word line driver circuit that can be included in SRAM memory of FIG. 2.
FIG. 5 illustrates an example of the waveforms for various signals used in the word line driver circuit of FIG. 4.
FIG. 6 illustrates an example of a diagram showing static noise margin for the bit cell of FIG. 3.
FIG. 7 illustrates an example of static noise margin versus VDD for the bit cell of FIG. 3 compared to known bit cells.
FIG. 8 illustrates an embodiment of a FinFET layout that can be used to implement the bit cell of FIG. 3.



DETAILED DESCRIPTION
Embodiments of SRAM memory devices are disclosed that include six transistor (6T) bit cells with bit lines pre-charged to 1⁄2 VDD and low VT pass transistors. The bit cells use 50% lower power during read and write operations since the bit lines swing 1⁄2 of the supply voltage (VDD) instead of full VDD. Additionally, the bit cells have faster write and pre-charge speeds since bit lines only slew 1⁄2 VDD. Bit lines driven with 1⁄2 VDD allow faster sensing since the cross coupled p-transistors as well as the cross coupled n transistors of a dynamic sense amplifier can be employed to provide gain. Use of 1⁄2 VDD instead of full VDD also reduces power supply noise due to less crowbar current as sensing resolves more quickly. The area required to implement the 1⁄2 VDD bit cells in a highly scaled technology is comparable to other 6T bit cells. The 1⁄2 VDD bit cells have similar or improved stability as other 6T bit cells. Since more transistors are employed in reading data from the bit cell to create a signal on the bit lines and more transistors are employed in sensing the bit line signal, there is a reduced chance of a defect in any of those transistors being fatal to the operation of the SRAM, resulting in a higher memory yield. Further the transistor sizes associated with the bit cell with 1⁄2 VDD bit lines provide a higher immunity to soft errors.
FIG. 1 illustrates a block diagram of an embodiment of a processing system 100 in accordance with the present invention that can include one or more processors 112, one or more SRAM memories 114, one or more other memories 116, one or more other modules 118, and an external bus interface 120 which may be bi-directionally coupled by way of bus 122. External bus interface 120 can be coupled to an external bus 124 which may be used to communicate information to and from system 100. SRAM memory 114 can be coupled to one or more integrated circuit terminals 126 which may be used to communicate information between SRAM memory 114 and circuitry (not shown) that is external to system 100. In alternate embodiments, computer processor(s) 112, other memory(s) 116, and other module(s) 118 may have one or more integrated circuit terminals (not shown) which may be used to communicate information to and/or from circuitry (not shown) that is external to system 100.
An embodiment of SRAM memory 114 includes an array of bit cells that may be Write accessed to store data and Read accessed to retrieve the stored data.
FIG. 2 illustrates a block diagram of components that can be included in SRAM memory 114 of FIG. 1 in accordance with the present invention including block decoder 201, row decoder 202, and memory array 203. Row decoder 202 provides global word line signals GWL to word line drivers 204, 206, 208. A selected one of the global word line signals provided is asserted. Block decoder 201 provides block select signals BS to block select drivers 205, 207, 209. The block select signal BS selects one of the block select drivers 205, 207, 209, which provides a block select bar signal BSB to its corresponding word line driver 204, 206, 208, for selecting and deselecting that word line driver. The selected word line driver selects a word line corresponding to the asserted GWL signal. The selected word line selects a plurality of bit cells coupled to that word line.
FIG. 3 illustrates a circuit diagram of bit cell 300 coupled to bit line BL, complementary bit line BLB, and pre-charge circuit 330 that are included in SRAM memory array 203 of memory 114 of FIGS. 1 and 2. Memory cell 300 comprises pass transistors 302, 304, pull-up PMOS transistors 306, 308, and pull-down NMOS transistors 310, 312. Pass transistors 302, 304 are low threshold voltage (VT) MOSFETS with a substantially lower VT than pull down transistors 310 and 312. Due to the low VT of pass transistors 302, 304, negative voltage applied to the gates of pass transistors 302, 304 is required to shut off current flow through pass transistors 302, 304.
PMOS transistor 306 and NMOS transistor 310 are coupled as a first inverter 314 with the source of PMOS transistor 306 coupled to supply voltage VDD and the source of NMOS transistor 310 coupled to ground VSS. The gates of transistors 306 and 310 are coupled to the input of inverter 314 and the drains of transistors 306 and 310 are coupled to the output of inverter 314. PMOS transistor 308 and NMOS transistor 312 are coupled as a second inverter 316 with the source of PMOS transistor 308 coupled to supply voltage VDD and the source of NMOS transistor 312 coupled to ground VSS. The gates of transistors 308 and 312 are coupled to the input of inverter 316 and the drains of transistors 308 and 312 are coupled to the output of inverter 316. The inputs and outputs of inverters 314, 316 are cross-coupled to one another and to the first current electrodes of low VT pass transistors 302, 304 at respective sense nodes SN and SNB. A bit line signal BL is coupled to the second current electrode of pass transistor 302 and a complementary bit line signal BLB is coupled to the second current electrode of pass transistor 304. Word line signals WL are coupled to the control gates of pass transistors 302, 304.
With a conventional 6-T bit cell with bit lines pre-charged to VDD, one of the bit lines must be switched for a power consumption that is a function of capacitance times the square of voltage (CV2). With the bit lines of bit cell 300 being pre-charged to 1⁄2 VDD, writing requires each of the bit lines to switch 1⁄2 VDD, one bit line to VDD and one bit line to VSS. Since each bit line of the bit cell 300 must switch only 1⁄2 VDD, the power is a function of 2*C*(1⁄2 V)2=2*1⁄4 *CV2=1⁄2 CV2, or one-half the power of the conventional cell. The data nodes SN and SNB store the data written to bit cell 300 across inverters 314 and 316. When a logic ‘1’ is written to bit cell 300, the node SN will be a logic ‘1’ or VDD and the node SNB will be a logic ‘0’ or VSS. When a logic ‘0’ is written to bit cell 300 the node SN will be a logic ‘0’ or VSS and the node SNB will be a logic ‘1’ or VDD.
Pre-charge circuit 330 comprises transistors 320, 322, and 324. The sources of transistors 320 and 324 are coupled to 1⁄2 VDD and the drains of transistors 320 and 324 are coupled to bit line signals BL and BLB respectively. The current carrying electrodes of transistor 322 are coupled to BL and BLB. The gates of transistors 320, 322 and 324 are coupled to a pre-charge signal PC. During pre-charge for either a Read or Write access, the bit lines are shorted together by transistor 322, returning both bit lines to 1⁄2 VDD. Transistors 320 and 324 keep the 1⁄2 VDD level from drifting over time. Since the bit lines are shorted together, there is no power supply current surge during pre-charge and pre-charge can be switched much faster than with a conventional 6-T bit cell that has a current surge on VDD during pre-charge.
With standard 6-T SRAM bit device sizes, the low threshold voltage VT associated with low VT pass transistors 302, 304 would improve write performance of bit cell 300 but would correspondingly degrade read performance due to a low signal noise margin associated with the low threshold voltage and small pull-up PMOS transistors 306, 308. To help offset the low signal noise margin, the drive strength or transconductance of PMOS transistors 306, 308 is selected to be approximately the same as the drive strength or transconductance of NMOS transistors 310, 312. PMOS transistors 306, 308 with approximately the same transconductance as NMOS transistors 310, 312 helps improve the stability or static noise margin of bit cell 300 given the low static margin associated with low threshold voltage for pass transistors 302, 304 while providing sufficient write margin. Additionally, the relatively strong P-transistors 306, 308 provide a strong current to counter soft error events, which attempt to destroy the ‘1’ level of the bit cell 300.
The low VT pass transistors 302, 304 along with the matching transconductance of the pull up transistors 306, 308 and the pull down transistors 310, 312, are sufficient to provide stability with the bit lines pre-charged to approximately 1⁄2 VDD. When the word line of bit cell 300 is selected with lines pre-charged to approximately 1⁄2 VDD, one of the pass transistors 302 or 304 will pull up on BL or BLB respectively, depending on the data stored on the nodes SN and SNB of memory cell 300. The other pass transistor 302 or 304 will pull down on BL or BLB respectively. Unlike a conventional 6-T SRAM bit cell which can only pull down, both sides of the bit cell 300 are active. During a Read access, the pass transistor 302 or 304 coupled to corresponding node SN or SNB with a stored ‘1’ will pull up on the corresponding bit line BL or BLB, while the other pass transistor 302 or 304 coupled to the corresponding node SN or SNB with a stored ‘0’ will pull down on its corresponding bit line. Since both sides of the bit cell 300 provide current to develop signal on the bit line, the bit cell is less sensitive to defects in any transistor of the bit cell 300, allowing memory 114 to have higher yield. In a similar way the sense amplifier (not shown) associated with bit lines BL and BLB will both pull up and pull down to amplify the differential signal on BL and BLB, which allows faster sensing and higher resistance to defects.
FIG. 4 illustrates a circuit diagram of a word line driver circuit 400 including word line driver 402 and block select driver 404 that can be included in SRAM memory 114 as respective word line drivers 204, 206, 208 and block select drivers 205, 207, 209 of FIG. 2. Word line driver 402 includes a first pass transistor 406, a second pass transistor 408, and latch circuit 409. Pass transistor 406 with a gate electrode coupled to VDD conducts a global word line signal GWL to a control gate terminal of pass transistor 408. The GWL signal varies between the supply voltage VDD and ground VSS. When the GWL signal is active high, pass transistor 408 conducts a complementary block select BSB generated by block select driver 404 to latch circuit 409. Latch circuit 409 includes a drive inverter 412 and a feedback inverter 410. The inverters 410, 412 are coupled between VDD and a negative voltage VN that is lower or less than ground VSS such that output signal WL swings between VDD and VN. When GWL is active high and BSB is active low drive inverter 412 receives the active low BSB signal and outputs a word line signal WL that is provided to bit cells coupled to the word line, such as bit cell 300 in FIG. 3. Feedback inverter 410 receives the word line signal WL and provides a feedback signal to the input to drive inverter 412, latching the output signal word line signal WL.
Block select driver 404 includes a first pass transistor 422, a second pass transistor 424, and latch circuit 425. Pass transistor 422 with a gate coupled to VDD conducts a block select signal BS from block decoder 420 to a control gate terminal of pass transistor 424. When the block select signal BS is active high, pass transistor 424 conducts a complementary word line clock signal WLCLKB to latch circuit 425 and pass transistor 408 for word line driver circuit 402. The WLCLKB signal is driven between the supply voltage VDD and ground VSS. Latch circuit 425 includes two inverters 426 and 428 coupled between VDD and a negative voltage VN that is lower or less than ground VSS. Inverter 428 receives the WLCLKB signal and outputs a word line clock signal WLCLK that is provided to the input of inverter 426.
In some highly scaled technologies, the inverters 410, 412, 426 and 428 may require series cascode devices in series with the pull up and pull down devices to avoid breakdown resulting from the larger than normal voltage across these devices due to the VN supply. In some highly scaled technologies pass transistors 406, 422 may have VTs substantially lower than other n-channel transistors of word line driver 402 and block select driver 404. For example, in some embodiments, the threshold voltage VT of pass transistors 406, 422 is approximately 30 to 50 percent lower than the threshold voltage of NMOS transistors 408, 424 of word line driver 402 and block select driver 404. Other suitable threshold voltages can be used for pass transistors 406, 422, however.
The operation of word line driver circuit 400 and block select driver 404 is now described with reference to FIG. 5, which illustrates an example of time waveforms for various signals used in the word line driver circuit 400 of FIG. 4. At the start a word line activation, the global word line GWL and block select BS signals are driven from ground VSS to the supply voltage VDD for the selected word line driver 402. The block select signal BS is coupled through pass gate transistor 422 to node A which is coupled to the gate of transistor 424. The global word line signal GWL is coupled through pass gate transistor 406 to the gate of pass transistor 408. After the global word line and block select signals reach VDD, the word line clock signal WLCLK is driven from the supply voltage VDD to ground VSS, which drives the complementary block select signal BSB from the supply voltage VDD to ground VSS. After the word line clock and complementary block select signals reach ground, the word line signal WL generated by latch circuit 409 is driven from a negative voltage VN to a positive voltage VDD.
The word line signal WL selects bit cell 300 and other bit cells on the word line signal WL allowing a differential signal to be coupled from bit cell 300 onto bit lines BL and BLB of bit cell 300, and in a similar way differential signals are coupled onto the associated bit lines of the other bit cells coupled to the word line signal WL. After the differential signals are developed on bit lines of the WL selected bit cells, the signals are each sensed by a corresponding sense amplifier (not shown). After the word line signal WL reaches the VDD voltage, the word line clock bar signal WLCLKB is tri-stated allowing the outputs inverters 410 and 428 to switch to negative voltage VN. The tristated word line clock bar signal WLCLKB follows the complementary block select signal BSB to the negative voltage VN. Once the signals on the bit lines of the selected bit cells including bit cell 300 are sensed with the global word line signal GWL remaining high, the word line clock signal WLCLK is driven back up to the supply voltage VDD. The parasitic capacitance between control gates and respective current electrodes in pass transistors 408, 424 self boosts nodes A and B at pass transistors 408, 424 to a voltage above VDD allowing a full VDD level to be coupled through pass transistors 408 and 424 to the signal BSB and the inputs to inverters 412 and 428 deselecting word line WL to the VN voltage. The VN voltage compensates for the lower VTs of the pass transistors 302 and 304 of bit cell 300 to fully turn off pass transistors 302 and 304. The GWL voltage remains active high until the word line WL is turned off to allow self boosting.
With reference to FIGS. 3 and 6, FIG. 6 illustrates an example of curves 600 showing static noise margin from a simulation of the bit cell 300 in a PDSOI (Partially Depleted SOD technology. The x axis is VSN or the voltage on node SN and the y-axis VSNB is the voltage on node SNB, both of bit cell 300. Static noise margin (SNM) is a measure of the stability of the bit cell during the read operation. The size of the openings 602, 604 in the curve is commonly measured as the SNM. Opening 602 corresponds to the left pass transistor 302, the right pull-up PMOS transistor 308 and pull-down NMOS transistor 312 of bit cell 300. Opening 604 corresponds to the right pass transistor 304, the left pull-up PMOS transistor 306 and the left pull-down NMOS transistor 310 of bit cell 300.
For some embodiments of the bit cell 300 of FIG. 3, the low threshold voltage pass transistors 302, 304 have a threshold voltage VT as low as 0 Volts. While the low threshold voltage makes the pass transistors 302, 304 much stronger for write operations, the low threshold voltage degrades the SNM. This occurs because voltage on node SN of FIG. 3 increases during the read operation for a ‘0’ value stored on node SN as the pass transistor 302 becomes stronger. However, bit lines pre-charged to VDD/2 as used for bit cell 300 weakens the pass transistor 302 and thus helps counter the effect of low threshold voltage of pass transistor 302. The SNM is also improved by pull-up PMOS transistors 306, 308 having approximately the same transconductance as NMOS transistor 310, 312.
Referring to FIGS. 3 and 7, FIG. 7 illustrates an example of static noise margin versus VDD for bit cell 300 in curve 702 compared to known bit cells in curve 704. The overall impact of bit cell 300 is estimated for FDSOI (Fully Depleted SOD devices, such as FinFETs. Curve 702 corresponds to pull-up PMOS transistors of comparable strength or transconductance to the pull-down NMOS transistor while the pass transistor threshold voltage VT is approximately 0 Volts and the bit line voltages are set to VDD/2. Curve 704 corresponds to a conventional SRAM cell from a PDSOI technology. The y-axis represents static noise margin (SNM) sigma and is an estimate of the number of sigma's that the SNM is away from 0 assuming normal statistics. The dashed line at Nsigma equals 5.2 corresponds to a minimum level that is needed to provide acceptable yield on large arrays of bit cells. Comparing curve 702 to curve 704, bit cell 300 achieves higher static noise margin while operating at much lower supply voltage VDD than previously known bit cell configurations represented by curve 704.
Referring to FIGS. 3 and 8, FIG. 8 illustrates an embodiment of a FinFET layout 800 that can be used to implement one half of bit cell 300 of FIG. 3 including two fins 804, 816 coupled to respective pull-up PMOS transistors 306, 308, two fins 808, 810 coupled to respective pull-down NMOS transistors 310, 312, and one fin 802 coupled to a first low threshold voltage MOSFET that is connected as pass transistor 302. A bit line is coupled to fin 802. Supply voltage VDD is coupled to fin 804 and fin 816. Fins 808, 810 are coupled to ground VSS. Word line signal WL is coupled to pass transistor 302. PMOS transistors 306, 308 are coupled to node SNB. NMOS transistors 310, 312 and fins 804, 806 are coupled to node SN. In some embodiments the low threshold voltage transfer devices may be depletion devices.
By now it should be appreciated that, in some embodiments, there has been provided, in some embodiments, a memory device that can comprise a plurality of bit cells. The bit cells include a first inverter including a PMOS transistor and an NMOS transistor, and a second inverter including a PMOS transistor and an NMOS transistor. The second inverter is cross-coupled with the first inverter. A first low threshold voltage metal oxide semiconductor field effect transistor (MOSFET) is coupled as a pass transistor to the first and second inverters. A second low threshold voltage MOSFET is coupled as a pass transistor to the first and second inverters. A threshold voltage of the first and second low threshold MOSFETS is substantially less than a threshold voltage of the NMOS transistors of first and second inverters.
In another aspect, a power supply voltage (VDD) can be coupled to provide approximately 1⁄2 VDD to bit lines during pre-charge operation.
In another aspect, transconductance of the PMOS and NMOS transistors in the first and second inverters can be approximately equal.
In another aspect, the first and second low threshold voltage MOSFETs can be configured to receive word line signals that are coupled below ground voltage.
In another aspect, the bit cells can be configured as FinFETs.
In another aspect, the first and second threshold voltage MOSFETs can have a threshold voltage (VT) of approximately zero volts.
In another aspect, the first and second low threshold voltage MOSFETs are depletion devices.
In another aspect, the bit cells can be static random access (SRAM) bit cells.
In other embodiments, a SRAM bit cell can comprise a first inverter including a PMOS transistor and an NMOS transistor, a second inverter including a PMOS transistor and an NMOS transistor. The first and second inverters are cross-coupled to each other. A plurality of pass transistors couple the inverters to bit lines. Approximately one-half of a supply voltage is provided to the bit lines during pre-charge operations.
In another aspect, transconductance of the PMOS and NMOS transistors in the first and second inverters can be approximately equal.
In another aspect, the plurality of pass transistors can comprise low threshold voltage devices.
In another aspect, the plurality of pass transistors have a threshold voltage (VT) of approximately zero volts.
In another aspect, the plurality of pass transistors can be depletion devices.
In another aspect, the PMOS and NMOS transistors can be FINFETS.
In another aspect, a first word line can be coupled to a control gate terminal of a first pass transistor of the plurality of pass transistors. A second word line can be coupled to a control gate terminal of the second pass transistor of the plurality of pass transistors.
In another aspect, a bit line can be coupled to a current electrode of the first pass transistor. A bit line bar can be coupled to a current electrode of the second pass transistor. The first and second transfer devices are NMOS transistors.
In another aspect, a second current electrode of a first pass transistor of the plurality of pass transistors can be coupled between a drain electrode of the PMOS transistor and the drain electrode of the NMOS transistor of the first inverter and to control gates of the PMOS and NMOS transistors of the second inverter.
In another aspect, a second current electrode of the second pass transistor of the plurality of pass transistors can be coupled between a drain electrode of the PMOS transistor and a drain electrode of the NMOS transistor of the second inverter and to control gates of the PMOS and NMOS transistors of the first inverter.
In other embodiments, a method can comprise applying approximately one-half of a supply voltage to a bit line signal and a bit line bar signal during a pre-charge operation for an SRAM bit cell. The bit line signal is coupled to a first current electrode of a first low threshold voltage transistor and the bit line bar signal is coupled to a first current electrode of a second low threshold voltage transistor. The first and second low voltage transistors are coupled as pass transistors to first and second inverters in the bit cell.
In another aspect, transconductance of PMOS and NMOS transistors in the first and second inverters is approximately equal.
Because the apparatus implementing the present disclosure is, for the most part, composed of electronic components and circuits known to those skilled in the art, circuit details will not be explained in any greater extent than that considered necessary as illustrated above, for the understanding and appreciation of the underlying concepts of the present disclosure and in order not to obfuscate or distract from the teachings of the present disclosure.
Moreover, the terms “front,” “back,” “top,” “bottom,” “over,” “under” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the disclosure described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.
Some of the above embodiments, as applicable, may be implemented using a variety of different information processing systems. For example, although FIG. 1 and the discussion thereof describe an exemplary information processing architecture, this exemplary architecture is presented merely to provide a useful reference in discussing various aspects of the disclosure. Of course, the description of the architecture has been simplified for purposes of discussion, and it is just one of many different types of appropriate architectures that may be used in accordance with the disclosure. Those skilled in the art will recognize that the boundaries between logic blocks are merely illustrative and that alternative embodiments may merge logic blocks or circuit elements or impose an alternate decomposition of functionality upon various logic blocks or circuit elements.
Thus, it is to be understood that the architectures depicted herein are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In an abstract, but still definite sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “operably connected,” or “operably coupled,” to each other to achieve the desired functionality.
Also for example, in one embodiment, the illustrated elements of system 10 are circuitry located on a single integrated circuit or within a same device. Alternatively, system 10 may include any number of separate integrated circuits or separate devices interconnected with each other. For example, memory 114, 116 may be located on a same integrated circuit or on a separate integrated circuit or located within another peripheral or slave discretely separate from other elements of system 100. Other modules 118 may also be located on separate integrated circuits or devices. Also for example, system 100 or portions thereof may be soft or code representations of physical circuitry or of logical representations convertible into physical circuitry. As such, system 100 may be embodied in a hardware description language of any appropriate type.
Furthermore, those skilled in the art will recognize that boundaries between the functionality of the above described operations merely illustrative. The functionality of multiple operations may be combined into a single operation, and/or the functionality of a single operation may be distributed in additional operations. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments.
In one embodiment, system 100 is a computer system such as a personal computer system. Other embodiments may include different types of computer systems. Computer systems are information handling systems which can be designed to give independent computing power to one or more users. Computer systems may be found in many forms including but not limited to mainframes, minicomputers, servers, workstations, personal computers, notepads, personal digital assistants, electronic games, automotive and other embedded systems, cell phones and various other wireless devices. A typical computer system includes at least one processing unit, associated memory and a number of input/output (I/O) devices.
Although the disclosure is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present disclosure as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present disclosure. Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all the claims.
Furthermore, the terms “a” or “an,” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to disclosures containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles.
Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements.

## Claims

1. A memory device, comprising:
a plurality of bit cells, wherein the bit cells include:
a first inverter including a PMOS transistor and an NMOS transistor;
a second inverter including a PMOS transistor and an NMOS transistor, wherein the second inverter is cross-coupled with the first inverter;
a first low threshold voltage metal oxide semiconductor field effect transistor (MOSFET) coupled as a pass transistor to the first and second inverters;
a second low threshold voltage MOSFET coupled as a pass transistor to the first and second inverters, wherein a threshold voltage of the first and second low threshold MOSFETS is substantially less than a threshold voltage of the NMOS transistors of the first and second inverters; and
a power supply voltage (VDD) coupled to provide approximately 1⁄2 VDD to selected bit lines during a pre-charge operation, wherein transconductance of the PMOS and NMOS transistors in the first inverter is approximately equal and transconductance of the PMOS and NMOS transistors in the second inverter is approximately equal.

2. The device of claim 1, wherein the first and second low threshold voltage MOSFETs are configured to receive word line signals that are coupled below ground voltage.

3. The device of claim 1, wherein the bit cells are configured as FinFETs.

4. The device of claim 1, wherein the first and second low threshold voltage MOSFETs have a threshold voltage (VT) of approximately zero volts.

5. The device of claim 1, wherein the first and second low threshold voltage MOSFETs are depletion devices.

6. The device of claim 1, wherein the bit cells are static random access (SRAM) bit cells.

7. A SRAM bit cell, comprising:
a first inverter including a PMOS transistor and an NMOS transistor;
a second inverter including a PMOS transistor and an NMOS transistor, the first and second inverters are cross-coupled to each other;
a plurality of pass transistors coupling the inverters to bit lines;
a supply voltage (VDD), wherein approximately one-half of the supply voltage is provided to the bit lines during pre-charge operations when the SRAM bit cell is selected, wherein transconductance of the PMOS and NMOS transistors in the first inverter is approximately equal and transconductance of the PMOS and NMOS transistors in the second inverter is approximately equal.

8. The bit cell of claim 7, wherein the plurality of pass transistors comprise low threshold voltage devices.

9. The bit cell of claim 8, wherein the plurality of pass transistors have a threshold voltage (VT) of approximately zero volts.

10. The bit cell of claim 7, wherein the plurality of pass transistors are depletion devices.

11. The bit cell of claim 7, wherein the PMOS and NMOS transistors are FINFETS.

12. The bit cell of claim 8, further comprising:
a first word line coupled to a control gate terminal of a first pass transistor of the plurality of pass transistors; and
a second word line coupled to a control gate terminal of the second pass transistor of the plurality of pass transistors.

13. The bit cell of claim 12, further comprising:
a bit line coupled to a current electrode of the first pass transistor; and
a bit line bar coupled to a current electrode of the second pass transistor, wherein the first and second pass transistors are NMOS transistors.

14. The bit cell of claim 8, further comprising:
a second current electrode of a first pass transistor of the plurality of pass transistors is coupled between a drain electrode of the PMOS transistor and the drain electrode of the NMOS transistor of the first inverter and to control gates of the PMOS and NMOS transistors of the second inverter.

15. The bit cell of claim 8, further comprising:
a second current electrode of the second pass transistor of the plurality of pass transistors is coupled between a drain electrode of the PMOS transistor and a drain electrode of the NMOS transistor of the second inverter and to control gates of the PMOS and NMOS transistors of the first inverter.

16. A method comprising:
applying approximately one-half of a supply voltage to a bit line signal and a bit line bar signal during a pre-charge operation for a selected SRAM bit cell, wherein the bit line signal is coupled to a first current electrode of a first low threshold voltage transistor and the bit line bar signal is coupled to a first current electrode of a second low threshold voltage transistor, wherein the first and second low voltage transistors are coupled as pass transistors to first and second inverters in the bit cell, wherein transconductance of the PMOS and NMOS transistors in the first inverter is approximately equal and transconductance of the PMOS and NMOS transistors in the second inverter is approximately equal.

17. The device of claim 1, further comprising:
a pre-charge circuit coupled to a bit line and a complementary bit line, the bit line is coupled to the first low threshold voltage MOSFET and the complementary bit line is coupled to the second low threshold voltage MOSFET, wherein the pre-charge circuit shorts the bit line and the complementary bit line together to 1⁄2 VDD during the pre-charge operation.

18. The device of claim 17, wherein the pre-charge circuit includes:
a first NMOS transistor having a first current electrode coupled to 1⁄2 VDD, a second current electrode coupled to the bit line, and a gate electrode coupled to a pre-charge signal;
a second NMOS transistor having a first current electrode coupled to 1⁄2 VDD, a second current electrode coupled to the complementary bit line, and a gate electrode coupled to the pre-charge signal; and
a third NMOS transistor having a first current electrode coupled to the bit line and the second current electrode of the first NMOS transistor, a second current electrode coupled to the complementary bit line and the second current electrode of the second NMOS transistor, and a gate electrode coupled to the pre-charge signal.